This invention relates to a high-pressure piston cylinder unit, especially an injection pump or an injection valve for an internal combustion engine.
In such a high-pressure piston cylinder unit, which is exposed to a great number of stroke cycles, as it is especially the case in an injection pump or an injection valve for an internal combustion engine, there is generally a piston present, which is carried in a cylinder bore, and which is exposed to a great pressure difference. The piston carried in the cylinder bore serves either to pump the fuel to be injected into the combustion chamber of the internal combustion engine, as in the case of an injection pump, or, in the case of an injection valve, it serves to open the injection valve when urged by the fuel that is to be injected, which is fed to it under high pressure, typically when the piston raises a needle valve coupled with it or made in one material piece with it from the valve seat of a needle valve and thus opens an injection cross section for the injection of the fuel into the combustion chamber of the internal combustion engine.
In a high-pressure piston cylinder unit of this kind it happens that, on account of ultimately unavoidable manufacturing tolerances, a deflection of the piston from its axis in the cylinder bore occurs, with the result that the pressure distribution is not uniform across the piston circumference on account of gap widths varying over the piston circumference, and a resultant radial force results therefrom which acts in the direction of the axial deviation. The unilateral urging of the piston in its guidance leads to wear on the surface of contact.
In the case of a common-rail fuel injection system, in which the fuel to be injected is held under high pressure in a reservoir and injected into the combustion chamber of the internal combustion engine by a fuel injector permanently fed with the fuel under high pressure, the danger of wear is especially high. In the case of permanently acting high pressures, the stress is made more severe by the fact that the radial forces resulting from the pressure forces act to the full extent during the entire stroke phase, unlike the case with a conventional injection system wherein the stroke takes place in any event partially in the pressure build-up phase, i.e., at pressures lower than the maximum injection pressure. Inasmuch as the piston serving for the operation of the injection valve of a common-rail fuel injector, which typically is coupled with the valve needle of the injection valve or is made of one piece of material with the latter, is permanently exposed to the applied high fuel pressure, when an off-axis deflection of the guidance of the nozzle needle to the nozzle needle seat, or of the piston in the cylinder bore, occurs, a permanent asymmetrical leakage occurs over the piston circumference. Furthermore, high radial forces intensifying the deflection, which are due to the high pressures, are present throughout the stroke, that is, especially right at the beginning of the stroke phase. These radial forces can lead to dulling or grinding and to great wear on the needle in the nozzle needle guide or on the piston in the cylinder bore.
DE 38 24 467 C2 discloses an injection valve for an internal combustion engine in which the valve needle is made bipartite with a hollow needle and a valve needle carried in an internal bore of the hollow needle. At its tip the hollow needle has a number of circumferential grooves which are spaced apart by approximately the same order of magnitude as the diameter of the valve needle and have a width and depth which correspond to about one-tenth of the valve needle diameter.
Furthermore, in MTZ 55 (1994) 9, p. 502, col. 3 and p. 511, col. 1, there is disclosed the use of titanium nitrite coatings for the pistons of fuel injection pumps for large diesel engines in order to prevent the xe2x80x9cseizingxe2x80x9d of the piston.
In EP 0 565 742 A1 methods are disclosed for the precision working of workpiece surfaces, especially the walls of bores in the cylinder of an internal combustion engine in which grooves arranged in a given pattern are produced in the surface by an irradiation treatment, especially by means of a laser, and are said to serve as a lubricant reservoir.
Lastly, in EP 0 419 999 B1 there is disclosed a method for the machining of surfaces subject to great friction in internal combustion engines, especially the inside surfaces of cylinders of piston engines, in which the surface is honed and finally subjected to a laser beam treatment, the laser treatment serving to vaporize protruding roughness tips or flaking in order to achieve a smoother surface.
The invention is addressed to the problem of devising a high-pressure piston cylinder unit, especially for an injection pump or an injection valve for an internal combustion engine in which there is less danger of wear due to axial deflection on a piston guided in a cylinder bore.
Furthermore, a method for the manufacture of such a high-pressure unit is to be provided by the invention.
By the invention, a high-pressure piston cylinder unit is created, especially an injection pump or an injection valve for an internal combustion engine, especially for a high stroke cycle rate, in which a piston guided in a cylinder bore is exposed on one end to a high pressure and thus a high pressure difference, while according to the invention fine grooves running parallel to one another at a small distance apart are formed in at least a portion of the guiding surface of the piston.
One advantage of the piston guiding surface made according to the invention is that a hydraulic pressure equalization is brought about on the circumference of the guide by the grooves, and thus any one-sided contact of the piston with the cylinder bore is prevented, or at least the pressing forces are reduced. As an additional advantage, the result is that the leakage flow is reduced after the piston is aligned centrally lengthwise of the piston guiding surface and thus the hydraulic efficiency of the unit is improved. A lessening of the leakage flow, however, is brought about also by the mere fact that the grooves running across the direction of the leakage flow act like a labyrinth seal. It is to be seen as a further advantage that the fluid present in the grooves wets the contact surfaces thereby achieving a lubricating effect.
The grooves formed in the guiding surface advantageously have a width b of between 5 and 100 xcexcm, preferably between 10 and 40 xcexcm.
The depth t of the grooves is advantageously between 3 and 50 xcexcm, preferably between 10 and 30 xcexcm.
The spacing a of the grooves amounts advantageously to between 0.05 and 1 mm, preferably between 0.1 and 0.5 mm, more preferably between 0.1 and 0.3 mm.
According to an advantageous embodiment, the width b of a groove is substantially the same as its depth t.
It is furthermore advantageous if the ratio of the depth t of the groove to the nominal diameter D of the guiding surface is between 1/200 and 1/1000.
According to an embodiment of the invention the grooves run circumferentially on the guiding surface.
According to a further development hereof the grooves can be made with a spacing a that varies lengthwise of the guiding surface.
According to another embodiment of the invention, the grooves run lengthwise of the guiding surface.
According to another embodiment of the invention, the grooves run at an angle to the length of the guiding surface.
According to a further development hereof the grooves can have a varying pitch lengthwise of the guiding surface.
According to another embodiment of the invention which is very advantageous from the manufacturing point of view the grooves are formed by a helical line.
This can be further developed if the helical line is multiple.
The helical line can have a pitch that varies lengthwise of the guiding surface.
According to still another embodiment of the invention, the grooves are made to cross at various angles to the length of the guiding surface.
This can be further developed by providing the grooves with a pitch varying lengthwise along the guiding surface.
In the above-mentioned embodiments it may be advantageous to provide the spacing a of the grooves lengthwise of the guiding surface such that it corresponds substantially to the working stroke of the piston in the cylinder bore.
In another embodiment of the invention, a plurality of the above-named patterns can be combined in forming the grooves.
According to an embodiment of the invention, the grooves are formed in an area of the guiding surface adjoining the high-pressure side of the piston.
As an alternative thereof the grooves can be formed over the entire area of the guiding surface.
According to an embodiment of the invention the grooves are formed in the circumferential surface of the piston that serves as a guiding surface.
Alternatively, or in addition thereto, the grooves can be provided in the cylinder bore serving as guiding surface.
The invention is especially valuable in a high-pressure piston cylinder unit which is the component of a fuel injector of a common-rail injection system in which the piston serves to operate the injection valve of the fuel injector, and in which the pressure difference is permanently applied to the piston. In such a component permanently exposed to the fuel pressure a constant axial deflection can occur, i.e., from the beginning of the movement of the piston in the cylinder bore, and therefore a substantial reduction of wear can be achieved by the invention with special advantage in this case.
In such a high-pressure unit serving as a component of a common-rail injection system, the piston is advantageously made in one piece with the nozzle needle of the injection valve, while the piston has a shoulder to which the fuel pressure of the common-rail injection system is permanently applied.
Advantageously according to the invention the grooves are formed on the circumferential piston surface serving as guiding surface where it adjoins the shoulder to which the fuel pressure is applied.
The method of the invention for the manufacture of a high-pressure unit of the invention provides that the grooves are produced by machining, for example by precision turning.
An alternative method which is especially advantageous is to produce the grooves by beam machining.
Such beam machining is advantageously performed especially by laser engraving.
An advantageous embodiment of the method of the invention provides such that, after the grooves have been produced, a lapping or fine grinding of the guiding surface is performed. But a precision working of the guiding surface can precede the production of the grooves.